Anterior dorso-lumbar spinal osteosynthesis instrumentation for the correction of kyphosis

ABSTRACT

The instrumentation comprises a pair of vertebral implants (1) provided with screws (4) for anchorage to two corresponding vertebrae located on each side of a damaged vertebra, and a plate (3) having two end portions (3a) for connection to the two implants, and equipped with serration (6, 15) for fixing it to the implants. The end portions (3a) of the plate have a rounded periphery and the instrumentation comprises fixing screws (50) having heads (51) which are rounded so that their surface is flush with the surface of the plate when they are anchored in the apertures of the implant and the apertures of the end portion (3a) of the plate. This arrangement ensures that the ends of the plate (3) do not come into contact with the vertebral discs when the angular position of the plate (3) relative to the implants varies, and reduces the overall size of the instrumentation.

The present invention relates to an anterior dorso-lumbar spinalosteosynthesis instrumentation and to a piston for placing thisinstrumentation in position, this instrumentation being adapted for thecorrection of kyphosis due to the destruction of one or two vertebralbodies.

There is disclosed in the French patent 89 12 187 (Publication No.2651992) an instrumentation (termed "implant" in this patent) whichcomprises a pair of vertebral implants provided with screws foranchorage to two corresponding vertebrae located on each side of adamaged vertebra, and a plate having two end portions for connection tothe two implants, this plate being provided with means for fixing theplate to the implants.

Now, it has been found that in certain cases the plate of this priorinstrumentation undergoes a slight flexion owing to the stresses towhich it is subjected, and consequently has a rigidity which issometimes insufficient.

Further, owing to their shape, the opposite ends of the connection plateare liable to come into contact with the adjacent vertebral discs whenthe implants turn relative to the plate, which may slightly damage thediscs by rubbing thereagainst.

Moreover, it has been found that the anchorage of the implants to thelumbar vertebrae of this prior instrumentation does not always have thedesirable solidity.

Lastly, the overall thickness of this instrumentation in the region ofthe end portions of the plate and the implants is relatively great andit has therefore been found desirable to reduce it.

An object of the invention is to provide an instrumentation which is soarranged as to avoid these drawbacks.

According to the invention, the end portions of the plate have a roundedperiphery and the means for fixing the implants to the plate are fixingscrews provided with rounded heads so that their surface is flush withthe surface of the plate when they are anchored in the apertures of theimplant and of the associated end portion of the plate.

The rounded shape of the periphery of the end portions of the plateeliminate any projecting part which would be liable to come into contactwith the vertebral discs during the relative rotations of the implantsand plate. Further, the fact that the fixing screws have a surface whichis flush with that of the plate when the instrumentation is placed inposition on the vertebral segment, markedly diminishes the overallthickness of the instrumentation relative to the instrumentationdisclosed in said patent.

According to other features of the invention:

if the instrumentation is intended for thoracic vertebral bodies, eachimplant is provided with two roughly diametrically opposed apertures forreceiving two corresponding anchorage screws for spongy bone;

if the instrumentation is intended for lumbar vertebral bodies, eachimplant is provided with three apertures for receiving three anchoragescrews for spongy bone. Consequently, the solidity of the anchorage inthe lumbar vertebrae is very substantially improved;

the end portions of the plate are provided with apertures whose edgesare conical and the end of the heads of the fixing screws has acorresponding conicity permitting a self-centring of the plate. Thecomplementary conicities of the head of the screws and the apertures ofthe implants moreover reduce the overall height of the instrumentation,on one hand owing to the aforementioned flush arrangement of thesurfaces of the head of the screws and the surface of the plate, and onthe other hand by the avoidance of certain connection members describedin said patent.

Another object of the invention is to provide a piston for placing inposition two implants which are part of the considered osteosynthesisinstrumentation, this piston being adapted to be handled by means ofdistraction or compression forceps.

Indeed, it has been found by experience that the mounting of implants bythe operating method described in said patent, employing solelydistraction forceps, is not always entirely satisfactory: thus, duringthe distraction and the compression, the vertebrae acted upon do notretain a certain freedom to rotate, which would however be desirable.

The invention therefore proposes a piston arranged in such manner as toovercome these drawbacks.

According to the invention, the piston comprises in combination:

a) a first element comprising a shaft extended by a body in which isprovided an annular groove for receiving a nose of a branch of theforceps, at least one radial opening for receiving a movable stud beingprovided in said body,

b) a second element in which is formed a longitudinal cavity permittingsaid second element to slide on the shaft of the first element, saidsecond element comprising a second body in which is provided an annulargroove for receiving a second nose of the forceps, and at least oneaperture for receiving a removable radial stud, said stud projectingfrom said first and second elements and being insertable incomplementary apertures in the implants,

c) means for adjusting the longitudinal position of the second elementon the support shaft of the first element and consequently the spacebetween the two studs respectively mounted on the first and secondelements.

The two elements of the piston are allowed a limited angular movementtherebetween which permits taking up a possible defective positioning ofthe implants.

Further, the ends of the removable studs screwed into the bodies of thetwo elements of the piston allow the implants freedom to rotate andconsequently, in the course of the distraction and compression, theyallow a certain freedom to the vertebrae being acted upon.

Further features and advantages of the invention will be apparent fromthe following description with reference to the accompanying drawingswhich illustrate two embodiments of the invention by way ofnon-limitative examples.

In the drawings:

FIG. 1 is a longitudinal elevational view to a larger scale of a plateof the spinal osteosynthesis instrumentation according to the invention,one end portion of this plate being mounted on an implant and these twoparts being provided with a screw which extends therethrough.

FIG. 2 is a top plan view of the plate shown in FIG. 1.

FIG. 3 is a top perspective view of the plate shown in FIGS. 1 and 2 andits screws fixing it to the implants.

FIG. 4 is a top plan view to a larger scale of a first embodiment of animplant of the instrumentation according to the invention, this implantbeing intended for anchorage in thoracic vertebrae.

FIG. 5 is a side elevational view of the implant shown in FIG. 4.

FIG. 6 is a top plan view to a larger scale of a second embodiment of animplant of the instrumentation according to the invention, this implantbeing intended for lumbar vertebrae.

FIG. 7 is a side elevational view of the implant shown in FIG. 6.

FIG. 8 is a perspective view of a device comprising an embodiment of thepiston according to the invention, two implants connected to thecorresponding studs of this piston, and distraction forceps the noses ofwhich are engaged in the annular grooves of the piston.

FIG. 9 is a perspective view of the part of the piston shown in FIG. 8provided with a shaft supporting the other part.

FIG. 10 is a longitudinal elevational view to a larger scale of thepiston shown in FIG. 8, the second element being moved closer to thefirst element.

The anterior dorso-lumbar spinal osteosynthesis instrumentationaccording to the invention will first of all be described with referenceto FIGS. 1 to 7.

The instrumentation comprises a pair of vertebral implants such as 1(FIGS. 4 and 5) or 2 (FIGS. 6 and 7), a plate 3 (FIGS. 2 and 3)providing a connection between the two implants 1 or 2, and screws 50for fixing the plate 3 to the two implants 1 or 2.

The plate 3 comprises opposite end portions or heads 3a which have arounded periphery and are provided with a central aperture 5. Providedaround the aperture 5 and on the surface of the head 3a adapted to cometo bear against the surface of the implants 1 or 2, are radialserrations 6 which extend to the periphery of the head 3, on a fullcircumference.

The fixing screws 50 have rounded heads 51 connected adjacent theirscrew threaded portions 53, to conical portions 52 adapted to fit intocorresponding conical surfaces 11 of the apertures 5. The bone anchoragescrews 4 for spongy bone each comprise a screw threaded stem 7 extendedby a conical surface of a head whose surface is flush with the surface3b of the plate 3, when the screws 4 are placed in position through theapertures 5 and central apertures 12 of the implants 1 and 2. Therounded surfaces 51 of the heads of the screws 50 are flush with thesurface of the plate 3 when the screws are anchored in the apertures(12, 5) of the implants 1, 2 and the end portions 3a of the plate.

The implant 1 is also provided with two apertures 13, 14 for receivingbone anchorage screws (not shown) on each side of the central aperture12, and is intended especially for thoracic vertebral bodies T4 to T12.The apertures 13 and 14 are spaced apart a distance which is as small aspossible so as to permit the fixing of the implant 1 on short vertebrae.The surface of the implant 1 adapted to receive the corresponding head3a of the plate 3 has radial serrations 15 which extend completelyaround the aperture 12 and on each side of the apertures 13, 14. Theserrations 15 of course match the serrations 6 so that the serrationsfit together. The opposite surface 16 of the implant 1 has a curvatureadapted to the anterior anatomy of the vertebral bodies against whichthe implant must be applied.

The body of the implant 1 comprises a lateral portion 1a which extendsbeyond the surface occupied by the serrations 15 and in which is formedan aperture 17 adapted to cooperate with the piston which will bedescribed hereinafter with reference to FIGS. 8 to 10. The aperture 14located on the posterior side of the vertebral implant 1 is inclined inthe anterior direction of the implant at an angle A which is preferablyabout 6°, as can be seen in FIG. 4 which shows an inclined fraction 14aof the wall of the cavity 14. On the other hand, the aperture 13 of thesecond screw, located on the anterior side of the implant 1, has itswall perpendicular to the upper surface of the implant 1.

The inclination of the axis A of the aperture 14 toward the anteriorside gives the surgeon confidence upon the insertion of the screws 4close to the medulla canal. The fact that the aperture 13 extends in adirection perpendicular to the upper surface of the implant 1 permits animproved bone anchorage and an increased safety against risk ofwrenching away.

The thickness e of the plate 3 varies in the same set of plates ofdifferent lengths in that the thickness of the plate 3 increases withincrease in its distance 1 between the centers or axes of the apertures5. This increase in thickness is determined in such manner as to providea constant rigidity of the plate from the shortest plate to the longestplate of the set. Thus, for a set of for example 33 plates, whose centerdistance 1 varies from 26 to 90 mm every 2 mm, the thickness of theplate varies progressively with the center distance from 3 mm to 4.6 mm.The rigidity of the plate in this way remains constant irrespective ofthe distance between the implants 1 or 2, so that there is no risk offlexion of this plate after the instrumentation has been mounted on apatient.

Experience has shown that the most unfavorable loading resulting in aninflexion of the plate, is at more than 500 MPa, but remains lower than600 MPa representing the yield strength of hyper hardened steel 316L,from which material the plate 3 may be advantageously made.

Further, the yield strength of cold rolled steel 316L for forging is1,000 MPa, steel TA6V having a yield strength of 900 MPa and titaniumT40 a yield strength of 340 MPa. Thus, titanium T40 cannot be employed,titanium TA6V on the other hand being well suited.

It was therefore possible to determine the rigidity of the plate fromthis first analysis. Then, in a manner known in the art, a rigidity wasdefined for the cases of previously considered loads. The data obtainedis shown in the following table:

    ______________________________________                                                    STEEL       TITANIUM                                              ______________________________________                                        inflexion     12 Nm/deg     46.6 Nm/deg                                       torsion       4.2 Nm/deg    2.3 Nm/deg                                        flexion       110 Nm/deg    60 Nm/deg                                         compression   7600 N/mm     4200 N/mm                                         ______________________________________                                    

This rigidity represents the plate having 90 mm between the centers oraxes. In starting with the results obtained in respect of the rigidityof a plate of 90 mm, the thicknesses of the plates of a series of plateswere defined with respect to their center distance to obtain a constantrigidity, which resulted for the set of plates in a thickness variationranging from 3 mm to 4.6 mm.

As already mentioned, the two rounded heads of the plates avoid anyprojecting part whatever the position of these heads relative to theimplants.

The lower face may comprise 48 serrations 6 arranged on 360° with aminimum pitch of 7°30" and a depth of 0.9 mm. The serrations of a 60°triangular shape perfectly fit into the serrations of the implants. Witha half-pitch offset relative to the implant it is possible to define a"0" position in which the axis of the plate 3 is perpendicular to theaxis of the implant 1 or 2. In order to avoid any projecting part, allthe edges are rounded.

The implant 2 is intended for lumbar vertebral bodies and is providedwith three apertures 19, 21, 22 for receiving three correspondinganchorage screws for spongy bone (not shown). The walls of the apertures19 and 21 are inclined in the anterior direction in the same way as thewall of the aperture 14, the aperture 22 being similar to the aperture13 of the implant 1. Thus, the two apertures 19, 21 are located on theposterior side of the implant 2, and the aperture 22 is located on itsanterior side.

The shape of the plate 3 has been calculated by the method of finishedelements. The purpose of this calculation is to optimize the shape andthe section of the plate by determining the distribution of the stressesand the rigidity of the plate. Other criteria are integrated, it beingnecessary for the plate to adapt itself to the anatomical morphology,

The calculations are based on the applied loads.

left-right lateral inflexion (10 Nm),

torsion (3.2 Nm)

flexion (10 Nm),

compression (100 daN).

The results are given in the form of Von Mises' isostress volumicdomains (for a 90 mm center distance plate, the largest element in theseries).

inflexion: T=587 MPa

torsion: T=118 MPa

flexion: T=210 MPa

compression: T=97 MPa.

The overall shape of the implant was defined with respect to themorphology of the vertebral bodies.

The position of the cavities receiving the spongy bone screws wasdefined on the basis of documents giving the heights, diameters andthicknesses of the cortical of the vertebral bodies from T4 to L4. Thisled to defining two implants the screw cavities of which are atdifferent distances apart.

Implant 1 having two apertures (FIG. 4)

d1=the overall size in height, i.e. the distance between the twoparallel sides C1 and C2 of the implant 1 (upper and lower sides in thedrawing).

d2=the distance between the tangent to the aperture 14 close to the sideC1 and parallel to the latter and the tangent to the aperture 13 closeto the side C2.

d3=the distance between the tangent to the aperture 14 close to theposterior side C3 of the implant 1 and parallel to this side and thetangent to the aperture 13 which is the closest to the anterior side C4of the implant.

Implant 2 having three apertures (FIG. 6)

d4=the overall height of the implant according to the same definitiongiven before, between the upper side C5 and the lower side C6 of theimplant 2.

d5=the distance between the tangent to the aperture 19 close to the sideC5 and parallel to the latter and the tangent to the aperture 21 closeto the side C6 and close to the latter.

d6=the distance between the tangent to the aperture 22 close to theanterior side C7 and parallel to the latter and the tangent to theaperture 21 close to the posterior side C8 and parallel to the latter.

The maximum distances d2, d3, d5, d6 are given in the following table asa function of the minimum and maximum heights of the vertebral bodies.The overall height d1 may vary from 13.7 to 20 mm and the overall sized4 of the implant 2 may vary from 20 to 32 mm.

    ______________________________________                                                      Implant-2  Implant-3                                                          apertures (1)                                                                            apertures (2)                                        ______________________________________                                        Maximum distance d2                                                                           11.5 mm                                                       Maximum distance d3                                                                           20.2 mm                                                       Maximum distance d5          17 mm                                            Maximum distance d6          20 mm                                            Vertebral body height                                                                         13.7 mm min  32 mm max                                        ______________________________________                                    

If the height of the vertebra is between 13.7 mm and 20 mm, an implant 1having two apertures is used. If this height is between 20 and 32 mm, animplant 2 having three apertures is used.

A minimum distance between the screws toward the interior is veryimportant since it must be ensured that the screws are not too closetogether so as to avoid risk of splitting the cortical.

The cavities (14, 19, 21) for the screws at the rear of the implant (1,2) are inclined at 6° toward the interior for two reasons:

1) give confidence to the surgeon when mounting the implant as theinclination avoids the medulla canal,

2) relative to the anterior screw, the assembly forms a triangularmounting ensuring an improved resistance to traction.

The 6° were defined by the fact that it was necessary to ensure that thelongest screws do not touch each other when passing through the othercortical, and that they are spaced far enough apart to avoid splittingthe other cortical, which would result in an imperfect fixing of theimplant.

As the adjustment between the screw and its cavity is very precise, itis possible to avoid the anchorage points as found on a known implant,which are liable to result in fractures and dangerous asperities for theorganism.

The choice was therefore made to retain a smooth surface in contact withthe bone by permitting the anchorage owing to the absence of clearancebetween the screw and the implant.

Description of the piston 23 (FIGS. 8 to 10

The piston 23 is adapted to permit the mounting of two implants 1 or 2adapted for handling with distraction or compression forceps 24, eachbranch 25 of which terminates in a nose 26.

The piston 23 comprises the following component parts:

a) A first element 27 having a shaft 28 extended by a body 29 in whichis formed an annular groove or recess 31 dimensioned to receive a nose26; at least one radial opening 32, namely two openings in theillustrated embodiment, are formed in the two parts 29a, 29b of the body29 on each side of the groove 31.

b) A second element 33 in which is formed a longitudinal cavity 34 inwhich the shaft 28 is slidable so that the element 33 is slidable on theshaft 28. The second element 33 comprises a second body 35 in which areprovided an annular groove 36 for receiving the second nose 26 of theforceps 24, and at least one aperture 37, namely in the illustratedembodiment two radial apertures 37 for receiving corresponding studs 38.The apertures 37 and the apertures 32 may be for example tapped forreceiving screw threaded studs 38 having a spherical head 39 whichproject from the bodies 29 and 35 after insertion in their respectiveapertures 32, 37. The spherical end heads 39 are adapted to be insertedin corresponding apertures 17 of the implants 1 or 2 (FIG. 8).

c) Means for adjusting the longitudinal position of the second element33 on the support shaft 28, and consequently the distance between twostuds 38 respectively mounted on the elements 27 and 33 in two of theapertures 32, 37 spaced apart a distance corresponding to that betweenthe implants 1 or 2.

In the illustrated embodiment, the aforementioned means comprise alongitudinal slot 41 provided in the shaft 28, closed at its oppositeends and open laterally on at least one side, a lateral pin 42, whichextends through the second element 33, having an end portion slidablyengaged in this slot 41. Thus, during the movements in translation ofthe element 33 on the shaft 28, the pin 42 can abut against either oneof the ends 41a, 41b of the slot 41 and thereby limit the travel of theelement 33 on the element 27. Lastly, the means for providing a relativeadjustment of the two elements 33, 27 are , completed by a device forclamping the two elements in a predetermined relative position. In theembodiment illustrated in FIG. 10, this device is formed by a screw 43engaged in a radial opening in the central part 35a of the body 35 so asto be capable of entering the slot 41 and coming to bear against alongitudinal flat surface 44 on the shaft 28 and thereby clamping thesecond element 33 in the chosen position on the shaft 28.

The lateral pin 42 may be a spigot screw whose spigot is slidable in thelongitudinal slot 41. It will be understood that the four tappedapertures 32, 37 are in alignment so as to be capable of beingpositioned on the implants 1 or 2, two studs 38 being respectivelyinserted in the elements 27 and 33, each one in either one of the twoapertures 32, 37 in accordance with the distance between the twoimplants 1 or 2 and therefore with the center distance of thecorresponding plate 3. Thus, in the embodiment shown in FIG. 10, twostuds 38 are placed in the apertures 32, 37 which are the closest toeach other. The maximum distance apart corresponds to a position inwhich the pin 42 abuts against the end 41b of the slot 41 and in whichthe studs 38 are respectively inserted in the apertures 32, 37 which arespaced the greatest distance apart.

The body 35 of the second element 33 has a length exceeding that of thebody 29 of the first element 27 and the annular groove 36 of the secondbody 35 is axially offset toward the body 29 of the element 27 so as toconstitute an asymmetric piston. Further, on each side of thelongitudinal flat surface 44 and between one end of the latter and thepart 29b of the body 29, the surface of the shaft 28 has cylindricalbearing surfaces 45 and 46 which are complementary to a cylindricalinternal surface 48 of the longitudinal cavity 34 of the second element33. This arrangement permits a relative rotation of the two elements 27and 33 in addition to their freedom of movement in translation relativeto each other (arrows shown in FIG. 8).

The forceps 24 and the piston 23 are temporarily interconnected by meansof an indexing system with the use of a ball retained by a spring, theseelements being known per se and not shown. This system permits retainingthe piston 23 between the branches or jaws 25 of the forceps 24 whileallowing the separation of the two elements without creating largestresses liable to hinder the utilization of the piston.

This manner of fixing permits retaining a freedom of rotation of theforceps 24 around the shaft 28 of the piston 23, which clears theoperating field and permits positioning the piston 23 upon the insertionof the studs 38 in the implants 1 or 2. The pair of forceps 24 isequipped with a rack (not shown) known per se which enables the piston23 to be temporarily clamped in position.

Thus the device for operating the piston 23 comprises two forceps,namely distraction forceps and compression forceps. This system is basedon the condition according to which, whether the action be distractionor compression, the surgeon effects the same movement, i.e. acompression of the hand which enables him to feel very well, by thepressure exerted, the force applied to the piston 23.

Apart from the aforementioned advantages, the last-mentioned advantagehas further advantages: for a defined and constant curve, the piston maybe used in all of the existing center distance variations (from 22 mm to100 mm). The relative freedom of rotation allowed the two elements 27,33 of the piston 23 during its use, leaves a certain amount of freedomto the vertebrae being acted upon in the course of the distraction andthe compression.

The asymmetry of the piston 23 also affords the following advantages:with the moving part, constituted by the second element 33, longer thanthe "fixed" part 27, and with the groove or recess 36 for connecting theassociated forceps 24 offset toward the central part of the piston 23,it is possible for the surgeon to work under the ribs. Further, thisasymmetry, combined with the release of the forceps 24 from the piston23, permits working in all positions, and the piston 23 can be used justas well by a right-handed person and a left-handed person.

The surgical method of mounting the instrumentation just described, bymeans of the piston 23 and forceps such as the forceps 24, is thefollowing:

two respective implants 1 or 2 are placed in position on two vertebraelocated on each side of a damaged vertebra by means of anchorage screwsof the spongy bone type (described in particular in the aforementionedFrench patent);

two studs 38 of the piston 23 are inserted in the correspondingapertures 17 of the implants 1 or 2;

the noses 26 of the branched 25 of the forceps 24 are engaged on thepiston 23 at the places provided for this purpose, i.e. in the grooves31 and 36, the second element 33 being freely slidable on the shaft 28;

a slight upper distraction is effected by opening the forceps 24;

a graft is placed in position between the two implants 1 or 2;

the forceps 24 are released so that the piston 23 returns to its initialposition;

the forceps are interchanged so as to use the compression forceps (notshown in the drawings and known per se), and the implants 1 or 2 aremoved closer together by means of the compression forceps and withcompression of the two adjacent vertebrae;

the distance between the two implants 1 or 2 is measured;

the piston 23 is clamped in this position by immobilizing its twoelements 27 and 33 with respect to each other;

the plate 3 of corresponding length is chosen and is placed in positionon the two implants 1 or 2 and the mounting is terminated by insertionof the fixing screws in the apertures 5 and 12 of the plate 3 and theimplants.

Many alternative embodiments of the invention may be envisaged. Thus, itis for example clear that the means for adjusting the relative positionof the elements 27 and 33 of the piston 23 may be different from those(42, 43) shown, the same being true of the arrangement of the studs 38.

What is claimed is:
 1. Anterior dorso-lumbar spinal osteosynthesisinstrumentation for the correction of kyphosis due to the destruction ofone or two vertebral bodies, comprising in combination:a pair ofvertebral implants; bone screws for anchorage of said implants to twocorresponding vertebrae located on each side of a damaged vertebra; aplate having two end portions for connection to said two implants; meansfor fixing said end portions to said implants; said two end portions ofsaid plate having a rounded periphery; and said fixing means includingfirst apertures defined in each of said implants, second aperturesdefined in said end portions of said plate, and fixing screws providedwith rounded heads so that the surface of said fixing screws is flushwith the surface of said plate when said fixing screws are anchored in acentral aperture of said first apertures of said implants and saidsecond apertures of the respective end portions of said plate. 2.Instrumentation according to claim 1, wherein:each of said implants isprovided with a rounded portion corresponding to said rounded peripheryof said two end portions of said plate; and said first apertures furtherinclude bone screw apertures at diametrically opposed locations on saidrounded portion, said bone screw apertures configured for receiving twocorresponding ones of said bone screws for anchorage of said implants.3. Instrumentation according to claim 2, wherein:each of said implantsincludes a posterior region and an opposite anterior region; and atleast one of said bone screw apertures is located in said posteriorregion and has an axis inclined in a direction toward said anteriorregion.
 4. Instrumentation according to claim 3, wherein each of saidimplants has a first surface, said inclination is 6° relative to a lineperpendicular to said first surface.
 5. Instrumentation according toclaim 1, wherein said first apertures further include three bone screwapertures for receiving three corresponding ones of said bone screws foranchorage of said implants.
 6. Instrumentation according to claim 1,wherein said second apertures of said end portions of said plate haveconical edge portions and said fixing screws have heads which defineconical portions corresponding to respective conical edge portions ofsaid second apertures in said end portions of said plate, therebypermitting self-centering of said plate.
 7. Instrumentation according toclaim 1, wherein each of said two end portions of said plate includes afirst surface within said rounded periphery and a rounded portion ofeach of said implants includes a second surface opposing said firstsurface, each of said first and second surfaces being provided withmating radial serrations distributed around the central aperture of eachimplant and an aligned aperture of said second apertures in therespective end portions of said plate for the passage of said fixingscrews.
 8. Instrumentation according to claim 7, wherein said serrationshave a minimum pitch of 7°30".